1. Field of the Invention
The present invention relates to an imaging apparatus that transmits light from an image forming optical system to an image sensor through an image fiber.
2. Description of the Related Art
There are various types of imaging apparatuses configured to transmit light from an image forming optical system through an image fiber and take an image with an image sensor. As seen in, for example, a fluorescent imaging apparatus, an industrial weak light detection apparatus, and an astronomical observation apparatus, there has been known technology to amplify weak light inside an image fiber and acquire a high brightness image, or the like. The image fiber is an optical unit capable of transmitting a two-dimensional optical image through a fiber flux having a plurality of optical fibers bundled together.
In an imaging apparatus configured to transmit light from an image forming optical system to an image sensor through an image fiber, the quality of a taken image is likely to be degraded due to various factors. Some methods have been known to improve the quality of such a degraded image. For example, when a difference in the refractive index between the core of an image fiber and a cladding portion is small, light seeps through the cladding portion, which causes a decrease in contrast. In order to address the problem, Japanese Patent Application Laid-open No. S61-280185 discloses a method in which light passing through a cladding portion is measured as light having average light intensity and the value of the light is subtracted from an entire screen to obtain a high contrast image. In addition, Japanese Patent Application Laid-open No. 2005-338341 discloses a method in which the interval between an image fiber and an image sensor in an optical axis direction is appropriately adjusted and the folding is reduced to allow the high quality of an image. Moreover, Japanese Patent Application Laid-open No. H9-312385 discloses a method in which the alignment direction of an image fiber and the alignment direction of an image sensor in a direction perpendicular to an optical axis are appropriately adjusted and moiré is reduced to allow the high quality of an image.
The present inventor has found the problem that the illumination unevenness of an image occurs, due to the fluctuation of the amount of received light for each of pixels, in an imaging apparatus configured to guide the light to an image sensor through an image fiber.
It seems that one of the reasons causing the fluctuation of the amount of the received light is attributable to a difference in the structure between the image fiber and the image sensor. In general, each of optical fibers constituting the image fiber and each of pixels constituting the image sensor are different in shape and size. For example, in many cases, the outgoing end face of the optical fiber is formed into a circle, and the pixel is formed into a rectangle. Therefore, it is difficult to make the optical fiber and the pixel correspond to each other on a one-to-one basis, and the positional relationship (overlapping degree) between the outgoing end face of the optical fiber and the pixel is different for each of the pixels. When the optical fiber is formed into a circle, a clearance is caused between the optical fibers even if the optical fibers are most densely bundled together. In addition, although the optical fiber includes a core and a cladding portion to efficiently transmit light, the cladding portion hardly allows the light to pass through. Accordingly, among an optical image formed by an image forming optical system, light formed at the clearances between the optical fibers or the cladding portions is not transmitted to the pixels of the image sensor. In other words, the illumination of the light guided to the image sensor is decreased by the clearances between the optical fibers and by the cladding portions, and the amount of the decrease in the illumination is different for each of the pixels depending on the positional relationship (overlapping degree) between the outgoing end face of the core of the optical fiber and the pixel. From the above reason, the output value of each of the pixels of the image sensor is fluctuated even if an image of a subject having even illumination is taken. This results in the occurrence of the illumination unevenness (brightness unevenness) of an image. Moreover, the intensity distribution of the light transmitted inside the optical fiber may not be even. In this case again, the illumination of the light received by the pixel is also different depending on the positional relationship between the outgoing end face of the optical fiber and the pixel, and thus illumination unevenness occurs.